1. Field of the Invention
The present invention relates to a method of recording halftone dot images and more particularly to improvement for obtaining halftone dot images having high resolution.
2. Description of Background Arts
It is known to increase a screen ruling (the array density of halftone dots per inch or that of single lines per inch) of halftone dot images in order to improve the resolution of printed matter for improving the readability of very small characters.
There is a problem in that the tone of printed matter having a screen ruling of 200 lines/inch or more is unstable. Generally, therefore the screen ruling cannot be increased beyond to 200 lines/inch or more, limiting the improve resolution.
Another method for improving the resolution of printed matter is the Bisutti method which is disclosed in Japanese Patent Publication No. 44-6404 and Japanese Utility Model Application Laid-Open No. 59-27546. In accordance with the method, two or more series of halftone dots with respectively different sizes are arranged regularly like a square lattice.
The halftone dot images according to Bisutti's method is appropriate for monochrome printing, but not for color printing. In the case of overprinting color-separated images for four colors in color printing, screen angles in the halftone dot images for respective color blocks are offset 30.degree. apart, except for the yellow block which is offset by 15.degree., to avoid the occurrence of moire. In Bisutti's method, it is impossible to set the screen angles in the halftone dot images for the color blocks a (except for yellow a) to the 30.degree., value because one halftone dot image has substantially two screen angles different from each other by 45.degree..
On the other hand, minute annular patterns occur periodically due to the aforesaid differences in screen angle in the conventional methods of printing a plurality of halftone dot images imprinted on the same sheet and having different screen angles. The annular patterns are called a rosetta moire, which often occurs in a relatively large region on a printed sheet. The rosetta moire presents the problem that, although not usually noticed due to its minuteness, once it is noticed it deteriorates the quality of the image. When the screen angles in four color-separated images are set to the same value to prevent the rosetta moire from occurring, the rosetta moire does not occur, the problem of "color misplacement" is liable to occur due to color misregistration. That is, when the mutual positional relation of color-separated images on the printing sheet is slightly misplaced the visible tone of the printed matter changes, and some deviations in screen angles may cause a moire having a noticeably large pitch due to the repetition of such tone changes.